Trithioborate rocket fuel



Feb. 7, 1961 A. ZLETZ ETAL 2,970,428 TRITHIOBORATE ROCKET FUEL Filed Oct. 25, 1952 INVENTORS: 000 R. Garmody y I Alex Z/efz ATTOR/V TRITHIOBORATE ROCKET FUEL Alex Zletz, Park Forest, and Don R. Carmody, Crete, Ill., assignors to Standard Oil Company, Chicago, lll., a corporation of Indiana Filed Oct. 25, 1952, Ser. No. 316,898 Claims. (Cl. Gil-35.4)

This invention relates to reaction propulsion. More particularly, it relates to liquid fuels for use in a bipropellant rocket system. Still more particularly, the invention relates to a method of rocket propulsion by the use of a nitric acid oxidizer and a hypergolic fuel, which materials react to generate gases at high pressure and high temperature.

Reaction propulsion is now being used for many purposes. For uses in military missiles it is preferred to use a fuel system which is not dependent on atmospheric oxygen, i.e., rocket propulsion. At the present time rockets-are used to assist the take-off of airplanes; this use is commonly known as J ATO or ATO.

The fuels used for this purpose may be liquid or solid. The liquid fuels are divided into the monopropellants and the bipropellants. The monopropellants decompose to give hot materials which provide the driving force for the rocket; a well-known monopropellant is nitromethane. The bipropellant fuels consist of a fuel proper and an oxidizer.

In the bipropellant system the fuel and the oxidizer are injected separately and simultaneously into the combustion chamber of the rocket motor. Ignition means may be supplied to initiate the combustion or the combustion may be spontaneous. The products of decomposition resulting from the reaction of the fuel and the oxidizer are discharged through an orifice provided at the exit end of the combustion chamber to produce the driving force.

Because of the possibilities of electrical and/or mechanical failure of the auxiliary methods of ignition such as spark or hot surface, it is preferred to use a propellant system which is self-igniting. A liquid fuel which is self-igniting, i.e., spontaneously combustible when contacted with an oxidizer, is. known as hypergolic fuel.

Temperature has a very important effect on the activity of hypergolic fuels. Many materials which are hypergolic at temperatures of about +70 F. lose this characteristic when the temperature is lowered. The temperature at the earths surface may vary from a high of about +120 F. to a low of as much as 65 F. The

temperatures encountered at high altitudes are often as low as 65 F. and frequently are as low as -100 F. For military purposes the rocket-di iven missile must operate satisfactorily over the range of about +120 F. to about -65 5 F. and preferably lower than .65 F. An object. of this invention is a method of reaction propulsion by the interaction of a hypergolic fuel and a nitric acid oxidizer. Another object is reaction propulsion by the interaction of a nitric acid oxidizer and a hypergolic fuel, which fuel contains appreciable amounts of essentially non-hypergolic hydrocarbons. Still another object isa method ;of rocket propulsion, which method is not dependent on auxiliary ignition means for initiating combustion at temperatures on the order'of --65 F. A particular object is the preparation of a novel class of compositions. The above objects and other objects which will become apparent in the detailed description have been above structural formula represents the same or difierent Patented .Feb. .7, 19$}:

achieved by the interaction of a nitric acid oxidizer which contains not more than about 20 weight percent of nonacidic materials and a novel fuel, which fuel has the empirical formula RRR"S B. In the formula B represents the element boron, S represents the element sulfur and R, R and R" represent the same or different hydrocarbon radicals selected fromthe group consisting of: aliphatic radicals containing from 1 to 8 carbon atoms, naphthenic radicals containing not more than 8 carbon atoms and monocyclic aromatic radicals containing not more than 4 substituent carbon atoms. A hypergolic fuel can be obtained by blending the above described, thioborate with an essentially non-hypergolic hydrocarbon in suitable proportions. Y Y

Nitric acid oxidizers are intended to include anhydrous HNO aqueous HNO anhydrous HNO which has been fortified with N 0 aqueous HNO which has been fortified with N 0 nitrogen tetroxide, mixtures of N 0 with NO, mixtures of N 0 with N 0 and mixtures of nitric acid and oleum. The usefulness of nitric acid as an oxidizer,'in conjunction with a hypergolic fuel, decreases'as the non-acidic material content of the acid increases. As the non-acidic material content increases the time for the reaction to begin becomes excessively long. (This time between the mixing and the start of the combustion is known as the ignition delay.)

The term non-acidic material is intended to include substances that do not add to the energy content of the system, i.e., are not fuels and are used solely for the purpose of loweringthe freezing point of the oxidizer. Water is the most commonly used freezing point depressant for nitric acid. By the addition of 10% of water it is possible to depress the freezing point of nitric acid from 44 F. to -81 F. However, lower freezing points are obtainable by adding to the acid an aqueous solution of potassium nitrate or sodium nitrate. A mixture consisting of 92% acid, 4% water, and 4% potassium nitrate has a freezing point of about F. It has been found that when using the defined thioborates of this invention that low temperature hypergolic activity is not obtainable when the nitric acid oxidizer contains more than about 10 Weight percent of non-acidic materials, although at moderately low temperatures about 20% of non-acidic materials can be tolerated.

Particularly suitable oxidizers are white fuming nitric acid (WFNA) which, in the commercial grade, normally contains between about 2 and 3% of water, and red fuming nitric acid (RFNA) which, in the commercial grade, normally contains between about 3 and 5% of water and between about 5 and 22 weight percent of N 0 Nitrogen tetroxideN O is a satisfactory oxdizer foruse above its freezing point. A mixture of N 0,; and nitrous oxide, as described in US. 2,403,932, isa satisfactory oxidizer for use at temperatures as low as -60 F. The term nitric acid oxidizer as used in this specification and in the claims is intended to include all of the compositions described above which contain not more than 20% of non-acidic materials such as water' point depressant (Hereinafter the word. thioboratewill be used inter,

changeably with trithioborate,) The. symbol R in the hydrocarbon radicals selected from the group consisting of: aliphatic radicals containing 1 to 8 carbon atoms, naphthenic radicals containing 8 or less carbon atoms, and monocyclic aromatic radicals containing 4 or less substituent carbon atoms. The term aliphatic is intended to include radicals that contain one or more un saturated linkages as well as the alkyl radicals. The term naphthenic is intended to include not only the cyclic radicals containing 3 or more carbon atoms in the ring but also substituted rings. The term monocyclic aromatic is intended to include the phenyl radical and also substituted phenyl radicals and also includes the presence of unsaturated linkages in a side chain. In the case of the ring compounds, it is intended to include the compounds wherein the sulfur is linked either to a ring carbon atom or to a substituent or side-chain carbon atom.

More effective as hypergolic fuels are the organic trithioborates wherein R represents the same or different hydrocarbon radicals selected from the group consisting of: alkyl radicals containing 1 to 4 carbon atoms, unsaturated aliphatic radicals containing 2 to 8 carbon atoms, cycloalkyl radicals containing 3 to 4 carbon atoms, and the aromatic radicals phenyl, tolyl, xylyl, ethylphenyl and vinylphenyl.

The preferred fuels are the alkyl trithioborates. The preferred thioborate fuel for low temperature operation is triethyl trithioborate. For moderately low temperatures, the preferred fuels are trimethyl trithioborate, triethyl trithIoborate or mixtures thereof.

The alkyl thioborates are in general clear, mobile, high-boiling liquids, they are fairly stable when exposed to elevated temperatures in the absence of air, but are extremely susceptible to hydrolysis by atmospheric moisture. Thus they are quite stable when stored in sealed containers such as stoppered flasks and stainless steel drums.

In general the alkyl thioborates have low freezing points, with a great tendency to supercool, i.e., remain liquid at temperatures below the true freezing poInts. Also, the minor amounts of impurities present in thioborates as prepared have a beneficial freezing point depressing quality. An amount of impurity sufficient to noticeably depress the freezing point of the pure thioborate does not have an appreciably adverse effect on hypergolic activity of the impure material.

It has been observed that the presence of minor amounts of hydrolysis products (and oxidation products) of the thioborates have no appreciable effect on bypergolic activity. It is intended to include within the scope of the invention the use of aliphatic trithioborates which contain minor amounts of impurities resulting from the preparation thereof, and those which contain minor amounts of products resulting from atmospheric exposure of the aliphatic trithioborates.

It has been found that hydrocarbons which are essentially non-hypergolic even at temperatures of about +120 F. can be blended with the defined aliphatic thioborates to produce a mixed fuel that is hypergolic with the defined nitric acid oxidizers. The other component blended with the aliphatic thioborate to form the mixed fuel should have a low freezing point, on the order of 70 F., in order to obtain a mixed fuel that is operable at low temperatures. When an essentially non-hypergolic hydrocarbon is used as the other component of the mixed fuel, the boiling point of said hydrocarbon has an effect on the hypergolic activity of the mixed fuel; it is preferred that the maximum boiling point of sa d hydrocarbon be below about 600 F.

Certain hydrocarbons, such as, shale oil fractions, olefins, some aromatic hydrocarbons, etc. are quite reactive with nitric acid oxidizers. When using these reactive hydrocarbons less aliphatic thioborate is needed in the blend to produce a hypergolic mixed fuel than is needed when the hydrocarbon is essentially non-reactive.

A superior mixed fuel can be made by blending a hypergolic fuel with one of the defined aliphatic thioborates. Examples of suitable hydrocarbons which are essentially non-hypergolic ar'e virgin naphtha, kerosene, heater oil, jet fuel, such as JP-3 fuel, benzene, toluene, xylene, etc.

The permissible dilution of the particular alkyl thioborate is dependent on the type of non-hypergolic hydrocarbon diluent. In general a higher dilution is permissible with lower boiling aromatic hydrocarbons such as toluene, xylene and benzene as the diluent.

Furthermore, the amount of dilution permissible is dependent on the proposed temperature of operation; the lower the temperature of operation, the less diluent tolerable.

When operating with alkyl thioborates at a tempera ture of about 60 F. the mixed fuel may contain as much as 50 volume percent of lower boiling aromatic hydrocarbons. At this low temperature of operation the mixed fuel may contain as much as about 35 volume percent of paraffinic hydrocarbon such as octane or JP-4 fuel. When it is desired to operate at low temperature with minimum ignition delay, the mixed fuel should contain not more than about 30 volume percent of low-boiling aromatic hydrocarbon or not more than about 20 volume percent of parafiinic hydrocarbon.

Trirnethyl trithioborate was prepared as follows: Boron trichloride was introduced into a 3-necked flzsk which contained methyl mercaptan in the approximate ratio of three 111015 of mercaptan per mol of boron trichl-oride. Also in the flask with the mercaptan was sodium methyl mercaptide in mol ratio to the boron trichloride of three to one. The mercaptide was used to react with any HCl formed in the reaction. However, the mercaptide was recovered apparently unreacted. After the addition of boron trichloride over a period of three hours at reflux temperature the reaction mixture was maintained at reflux temperature (about 10 C.) for 4%. hours. The cooler was removed from the flask and the flask permitted to reach room temperature while stirring. The pressure in the flask was progressively decreased by withdrawing gaseous materials therefrom, ambient temperature being maintained for the fractionation process. Two overhead fractions. both of which were hypergolic with white fuming nitric acid at ambient temperature, and a sodium mercaptide residue were obtained. The lowerboiling overhead fraction was refluxed at sub-atmospheric pressure for about eight hours to carry the reaction as comple ely as possible to the formation of trimethyl trithioborate and the resulting product was subjected to distillative fractionation to obtain seven frac-.

tions. The last, least volatile fraction analyzed as follows:

Actual, Theoretical percent Trimethyl 'Irlthioborato Hydrogen 6. 15 5. 96

The ultimate analysis and the theoretical are so close together that this product is trimethyl trithioborate.

The other physical characteristics of the trimethyl trithioborate are freezing point, 3.5-4.0 C.; boiling point, zagint, 59 C. at 2 mm. Hg.; specific gravity, 1.09 at Triethyl trithioborate was prepared as follows: Ethyl mercaptan was cooled to -65 C. in a 3-necked flask provided with a Dry Ice condenser and a motor-driven stirrer. Gaseous boron trichloride was passed into the flask in an amount slightly less than the theoretical. The contents of the flask were allowed to come to room temperature and were stripped of ethyl mercaptan. The

material in the reaction flask fumed strongly whener'tQ posed to air. 7 V

These materials were placed in a bomb and were treated with add tional amounts of ethyl mercaptan while being maintained at a temperature of H C. The material was removed from the bomb and was found to give a slight qualitative test for chlorine. V V

This material was given a further treatment with sodium ethyl mercaptide in order to obtain a product that did not fume when exposed to the air. This product was distilled to produce a fraction which was hypergolic with 70% nitric acid at room temperature. Ultimate analysis of this fraction indicates that triethyl trith'ioborate had beenprepared. The physical characteristics of the triethyl trithioborate are freezing point, 46 C.; boiling point, 64-67 C. at 0.5 mm. of Hg; specific gravity, 1.06 at 24 C. 1

Triethyl trithioborate supercools readily and is quite fiuid at -79 C. When exposed to air some solid products were produced, The resulting liquid mixture had a freezing point of 48" C. I

The following tests illustrate the hypergolicactivity of the methyl and ethylthioborates. The hypergolic fuels known to the art are also listed for purposes of comparison.

Test 1 In this test the ignition characteristics of the oxidizer compositions were studied using a drop test'method. This method utilizes a test tube, 1 in. X4 in., containing 1 ml; of oxidizer. The fuel is added dropwise into .the test tube by means of a syringe calibrated in 0.01 ml. markings." Usually 0.1 ml. of fuel. is added per test; however, the fuel usage may vary between 0.01 and 0.2 ml per ml. of oxidizer. Low temperature tests were carried out by cooling the test tube and the oxidizer contained therein 'to the desired temperatureby means'of a Dry Ice-chloroform bath;.a. drying tube inserted into the top of the test tube'excluded moisture. 'The fuel was cooled separately to the desired temperature. By super cooling, i was possible to carry out tests at temperatures below the freezing point of the fuel and of the oxidizer. The time elapsing' between the addition'of the fuel to the oxidizer and ignition thereof.the ignition delay-was determined visually as either: very short, short, ignition or negative. A very short ignition delay corresponds to substantially instantaneous ignition.

(a).ln this group of runs the temperature of both oxidizer and fuel was held at +75. F. and the ignition delays were noted for the various oxidizers.

Fuel 94% HNO; 51 253 N104 Trimethslm Veryshort- Short.-- Ignition. Triethyl dn ...do Short. Frzrt'urylfl Neg".-. Neg. Aniline Short Neg-- Ignition.

(b) In these runs the temperature was held at 70 F.

Test 2 For a more accurate determination of ignition delay, a series of runs were conducted using an apparatus which permitted determination of ignition delay in terms of milliseconds. The ignition delay herein is the length of time elapsing between the mixing of the fuel and the oxidizer and the start of the spontaneous combustion reaction. These tests were run at two temperatures, +75 F.

and -'4l0 F. the oxidizer was WFNA (see Test 1 )L" In the tabulation below, ignition delays are in milliseconds.

Ignition Delay, intseconds Fuel Triethyl 6 17 In this test there was determined the maximum permissible dilution of trimethyl trithioborate with n-octane at various temperatures. The oxidizer in this test was 94% HNO F. Ignition Volume Percent n-Octane Delay Neg.

Short.

Very Short. 40 Short.

Test 4 In this test there was determined the maximum permissible dilution of toluene with n-octane at various temperatures. The oxidizer in this test was 94% HNO F. Ignition Volume Percent n-Octane Delay 46- -50 Very Short. 35 -52 D0.

I can usefully utilize the fuels'described herein. Referring to the drawing, vessel llr-contains a quantity of inert gas under high pressure; nitrogen or helium is a suitable gas. The gas is passed through line 12, through regulatory valve 13 which passes the gas into line 14 at a constant pressure. From'line 14, the gas is passed-into line.16 which is connected to the vessels containingthe fueland the oxidizer. Vessel 17 contains the oxidizer; the pressure of the gas from line 16'forces' the oxidizer through line 18, through solenoid actuated throttling'valve 19, through line 21, and through injector. 22 intocombustion chamber 23. Combustion chamber 23 is provided with an exit orifice 24. Vessel 26 contains the main supply of fuel. The gas pressure forces the fuel out of vessel 26 through line 27, through solenoid actuated valve 28, and through line 29 to vessel 31. Vessel 31 may be used to' contain a special starter fuel or an additional amount of the main fuel. The pressure in line 29 forces the contents of vessel 31 through line 32, through solenoid actuated throttling valve 33, through line 34 and through injector 36 into combustion chamber 23. The injectors 22 and 36 are so arranged that the streams of liquid 'violently impinge and thoroughly intermingle. The rethis fuel. The oxidizer consists of a commercial redfuming nitric acid. The ratio, on a weight basis, between the oxidizer mixtureand the fuel may be between about 1.5 and 5.0. In this example 3 lbs. of oxidizer composition are used per pound of fuel. The missile is launched by activating the solenoids on valves 19 and 33. The oxidizer and the fuel are forced into the combustion chamber by the pressure of helium gas. Combustion takes place almost instantly and the rush of hot gases through orifice 24 hurtles the missile toward the target.

The walls of the combustion chamber become very hot from the heat of the burning gases generated by the reaction of the fuel and the oxidizer. This hot surface, and the mass of hot gases in the chamber, has a pronounced favorable effect on the self-ignition characteristics of the fuel and oxidizer. Many fuels which are non-hypergolic at the temperature existing in the fuel tank of the rocket unit are rapidly hypergolic in the extremely hot combustion chamber. For economy of operation, a fuel that is hypergolic at very low temperatures may be used to initiate the combustion in and to start the cold reaction motor; the use of this starter fuel may be continued until the hot gases generated have heated the combustion chamber to a high temperature; at this point the flow of the starter fuel can be stopped and a cheaper,

although not as highly hypergolic, or even a non-hypergolic fuel, can be utilized for the continuous operation of the reaction motor.

The use of a starter fuel in conjunction with another type of main fuel is particularly advantageous when very high velocities are necessary. Certain fuels whose decomposition products are of relatively low molecular weight are used for high velocity purposes because of the very high thrust developed by these fuels. An airto-air missile usually has a relatively short combustion chamber and this fact limits the fuels that can be used for high thrust operation. Turpentine is an excellent high thrust fuel for this use. The turpentine is stored in vessel 26 and valve 28 is closed. About 4 lbs. of RFNA oxidizer are used per pound of turpentine. Triethyl trithioborate is used as the starter fuel and is stored in vessel 31. Only enough starter fuel to heat up the combustion chamber is needed; in this case 0.1 second of operation. The missile is launched by activating the solenoids in valves 19, 2S and 33. The turpentine forces the starter fuel into the combustion chamber where the oxidizer and the starter fuel ignite and heat up the chamber. Without interruption, the turpentine follows into the heated chamber and burns to give the very high thrust reaction.

Thus having described the invention, what is claimed is:

1. A rocket propulsion method, which method comprises injecting separately and simultaneously into the combustion chamber of a rocket motor a hypergolic fuel consisting essentially of an alkyl trithioborate having the empirical formula, R S B wherein B represents the element boron, S represents the element sulfur and R represents alkyl radicals containing from 1 to 4 carbon atoms, and a nitric acid oxidizer which contains not more than about 20 weight percent of non-acidic materials, in an amount and at a rate sufficient to initiate a hypergolic reaction with and to support combustion of the fuel.

2. The method of claim 1 wherein said oxidizer is white fuming nitric acid.

3. The method of claim 1 wherein said oxidizer is red fuming nitric acid.

4. The method of claim 1 wherein said fuel is trimethyltrithioborate.

5. The method of claim 1 wherein said fuel is triethyltrithioborate.

6. A method of initiating combustion in a rocket motor, which method comprises injecting separately and simultaneously into the combustion chamber of the rocket motor, triethyltrithioborate and red fuming nitric acid, in an amount and at a rate sufficient to initiate a hypergolic reaction with an to support combustion of the trithioborate.

7. A rocket propulsion method, which method comprises injecting separately and simultaneously into the combustion chamber of a rocket motor, (I) a hypergolic mixed fuel consisting of (a) a liquid essentially non-hypergolic hydrocarbon boiling below about 600 F. and (b) an alkyl trithioborate having the empirical formula R S B wherein B is boron, S is sulfur, and R is an alkyl radical containing from 1 to 4 carbon atoms and (2) a nitric acid oxidizer containing not more than 20 weight percent of non-acidic materials. in an amount and at a rate sufficient to initiate a hypergolic reaction and to support combustion of the mixed fuel.

8. The process of claim 7 wherein said hydrocarbon is toluene.

9. The process of claim 7 wherein said hydrocarbon is n-octane;

10. A novel composition of matter consisting essentially of a mixture of not more than about 50 volume percent of a hydrocarbon oil and the remainder essentially a trialkyl trithioborate wherein each alkyl group contains 1 to 4 carbon atoms.

7 References Cited in the file of this patent UNITED STATES PATENTS 2,160,917 Shoemaker et al. June 6, 1939 2,266,776 Leum Dec. 23, 1941 2,526,506 Rogers et al. Oct. 17, 1950 OTHER REFERENCES Journal of Space Flight, vol. 2, No. 1, January 1950, page 9. 

1. A ROCKET PROPULSION METHOD, WHICH METHOD COMPRISES INJECTING SEPARATELY AND SIMULTANEOUSLY INTO THE COMBUSTION CHAMBER OF A ROCKET MOTOR A HYPERGOLIC FUEL CONSISTING ESSENTIALLY OF AN ALKYL TRITHIOBORATE HAVING THE EMPIRICAL FORMULA, R3S3B WHEREIN B REPRESENTS THE ELEMENT BORN, S REPRESENTS THE ELEMENT SULFUR AND R REPRESENTS ALKYL RADICALS CONTAINING FROM 1 TO 4 CARBON ATOMS, AND A NITRIC ACID OXIDIZER WHICH CONTAINING NOT MORE THAN ABOUT 20 WEIGHT PERCENT OF NON-ACIDIC MATERIALS, IN AN AMOUNT AND AT A RATE SUFFICENT TO INITATE A HYPERGOLIC REACTION WIITH AND TO SUPPORT COMBUSTION OF THE FUEL.
 10. A NOVEL COMPOSITION OF MATTER CONSISTING ESSENTIALLY OF A MIXTURE OF NOT MORE THAN ABOUT 50 VOLUME PERCENT OF A HYDROCARBON OIL AND THE REMAINDER ESSENTIALLY A TRALKYL TRITHIOBORATE WHEREIN EACH AKLYL GROUP CONTAINS 1 